Hi everyone , where should i put the deepest part of the concave as a front footed surfer? Does it even make a differnce being front foot or back footed as to where the
concave is deepest?
Hi Miguelito - I think it’s open to discussion depending on where you place the concave and it’s purpose. If it’s in the nose for nose riding (not that a concave is mandatory on a noserider) it might be different from one placed under your feet in a normal riding position. In any case, I’d guess that most concaves are deepest approximately centered in their length.
Under your feet. Whether on the nose or in the middle of the board.
You shouldn’t really think about where to the deepest part should be, you should rather think about how the concave changes the rocker line.
Yes, I learned yesterday exactly what Hans is saying. I have a 8/2 rounded pintail singlefin that I made, it rides great and works in almost all the types of waves I surf. It is almost 22" widw, 2 7/8" thick and has the slightest belly to flat, V out tail. I love the way it works.
For fun, I just finished making another in the same outline, but this one with a little less nose rocker and (what I wanted to be) a slight concave at the nose back to about 1/3 length… I really just wanted to change it up to see what it would feel like.
Here’s the deal. I didnt know how to do the concave, so I started planing at the stringer to “lower” it to accomodate concave. Things started looking funky. I effectively pretty much flattened the rocker out of the front, what’s worse is didnt ever get the transition right at the back of the concave. The result was basically a microscopic “kink” in the length of the stringer.
The new version rides poorly. At a full 8’ and 22" wide, it is hard to get into waves! It surfs alright, even pretty good at speed, working more or less off the back 3rd of board, but it you step or weight forward at all, Its like putting on the brakes! Literally, there is someting happening there hydrdynamically that is very very wrong. It is frustrating, because the board feels pretty good at top speed…but when I push the nose down at all, the thing fairly stops and bucks you off! Anyway, my botched concave made this board hard to catch waves on, and almost impossible to flat-out trim. I didnt consider the rocker enough when I was gouging away, doing concave the wrong way.
So, all in all, be careful with the rocker as Hans just said. I would read alot and ask alot of questions before I started Good Luck!
Hey could I see a pic of that with a straight edge across it. Just pure curiosity.
Under your centre of weight is the guidance I’ve always been given. Ends up somewhere near mid of rocker low point and front of side fins in a typical multifin shortboard.
Hey McDing, turns out I’m as shitty at taking pictures of concaves as I am at shaping them… anyway, putting a straight-edge on it indicates to me the the concave is probably too severe, and does not run back naturally. Thats my estimation of why it “brakes”. Essentially, theres a kink in the stringer!
This was a project that got interrupted and moved around quite a bit, and took a long time to complete. I originally just wanted to make a copy of my rpt with different tail, flatter rocker. I don’t remember my motivation in adding the concave! I know that I wanted it to ride dicernibly different from it’s rounded pin companion. In that respect, I got what I asked for. There should be a saying about that…
An excerpt from our Surfboard Design Guide
Concave Bottoms
The most definitive and conclusive statement ever made about concave bottoms is this: “Make no generalizations about concaves!” Concaves are likely the most complicated, questioned and debated element of surfboard design theory, yet they are probably the most commonly utilized design feature on modern surfboard bottoms. Concaves come in many shapes and sizes, have a number of effects on performance, and are used on most surfboard types in different variations and combinations. Unfortunately, most shapers have little idea about how concaves work, their knowledge of the design element restricted to feedback from riders, other shapers, personal perceptions, and the utter void of scientific research on the topic that is available, applicable, and comprehendible by general surfing public. With this in mind, most of what we assume can be attributable to concaves has been deduced from what we have indeed learned as a shaping community over the years.
Redirection of Flow
As mentioned earlier, bottoms that incorporate concave designs change the direction of laminar flow of water under the board from angular, diffused flow back and toward the rail away from the stringer, to more channeled flow following the path of the concave. If the perimeter of the concave narrows and curves toward the stringer, as it would from the wide point back, water will encounter resistance against the aft section of the concave, creating a force against the downturned rail edge which pushes not only up (lift), but back and out as well, inducing a degree of drag. But this phenomenon is what gives some riders a sense “bite” or added hold in steep faces or hard turns.
If water is coming in from both sides of the board, the flow would have to converge, creating an area of high pressure and turbulence in the area of convergence. This convergence of flow is mitigated by the acceleration of tail rocker, but carefully manipulated, can also produce a sense of added lift and projection. In large surf, however, this added lift can become problematic, and can lead to a lack of control some riders refer to as “slippery” or “squirrely.” The likely cause of this sensation is the area of turbulence under the board, somewhere between the rider’s feet. To minimize flow convergence, shapers often turn to the use of double concaves in the aft section, which helps keep the flow from both sides of the template separate. A double concave is simply two parallel concaves on either side of the stringer.
Most modern performance thrusters use some kind of single to double concave setup, where the single concave is shaped first, and the double is shaped within the single. This puts the bottom rail edge lower than the stringer throughout the entire concave array. Typically, the single concave starts 12-18 inches from the tip of the nose, gradually deepening until it reaches its maximum depth somewhere between the wide point of the board and the midpoint between the wide point and the leading edge of the rail fins. The single then gradually fades out to flat or vee behind the trailing edge of the center fin. The double concaves can begin anywhere, but typically begin to fade in at the point where the single concave is deepest. The double concaves are usually deepest near the leading edge of the rail fins, then either quickly fade to flat or vee at the trailing edge of the center fin, or are run right out the tail block, the latter being more common on fish type boards. Many riders report that changing the length of the double concaves changes the feel of the board – lengthening the doubles gives a greater sense of control at speed and added drive. Shortening the doubles gives a greater sense of lift and responsiveness, but a lesser degree of control at speed. This might be explained by the fact that the further forward the doubles extend, the more the flow of water coming through the single concave is kept separated, reducing the turbulence caused by flow convergence as the template narrows.
Rocker Alterations
Because concaves remove material from the bottom of the board, but leave the rail line untouched, all concaves flatten areas of the bottom of the board, either along the stringer, in the case of single concaves, or on either side of the stringer, in the case of double concaves. Flattening the bottom rocker of the board reduces bottom curvature, creating flatter planing area(s) that can help the board plane earlier at lower speeds, and reach higher top end velocities. The deeper and longer the concave(s), the generally flatter the bottom planing areas will become, relative to rail rocker. Obviously, a deep single concave flattens more area than a double concave alone. The single-to-double concave can add even more flattened area than either the single or the double, if the double concave is shaped into a pre-existing single, further flattening an already flattened curve. In any case, this relative increase in rail rocker line allows the board to maintain its turning radius, while providing flatter planing bottom areas, giving the rider a greater sense of both speed and maneuverability.
Rail Presentation
Often overlooked, and sometimes considered insignificant, concaves change the way the rail is presented to oncoming water flow. The downturned bottom rail edge is considered by some to be more responsive to rail-to-rail pressure changes, as the edge “bites” or “grabs” sooner when the rail is weighted. Compared to flat or convex bottoms, which facilitate release, concaved bottoms help contain flow, conserve energy, and provide a harder rail curve along the bottom of the board that drives the rail deeper into the water due to the change of the plane of the bottom of the board near the rail. Proficient surfers use this feature, along with the concave’s ability to capture flow, as they pump the board for speed, driving off the downturned rail, and projecting off the flex of the board, the flex of the fins, and as they weight and unweight the board. Flat and convex bottoms generally lack the ability to create speed and drive through pumping, so the effect is limited to whatever the rider can get out of flex and fins.
How a concave design changes the plane of the bottom, and how the rail is presented while in trim, during a turn, or while pumping, also affects the water line along the bottom. Due to the curvature of a board’s rocker, there is a water line, usually at some angle, from the inside rail, across the bottom of the board, to the outside rail, that separates the “wetted bottom” from the “un-wetted bottom” under the entry rocker section of the board. Concaves cause this water line to shift slightly back through the concaved areas, as the bottom is lifted compared to the rail. Whether this shift is significant or not is unclear. However, even subtle changes should be considered to have possible affects on board performance when attempting to factor in all possible variables contributing to a more complete and comprehensive theory of bottom design.
I’ve noticed a similar lurching effect on my Fish and Mini Simmons when I have a belly in the nose to a deep-ish concave through the last 2/3 of the board. If I try to ride up close to the nose of the board it plows water and comes to a grinding halt. My theory is that the transition from convex to concave is a lot of curve for the water to hold onto and causes lots of drag which, ultimately, pulls the nose down even further into the water.
But all of these boards are 5’ 4" and under, so maybe different dynamic. They work insane surfing from either the back or the middle of the board.
Mr. Bill Thrailkill gave me some advice to bottom contours, take from it what you will. He said: “whatever the bottom contours are, make them unsee-able to the eye.”
I’ve been taught to count sandpaper strokes to get even flow and depth in concaves.
Say four full length; 3 in mid 2/3; and 2 and mid 1/3 for concave in vee.
Bit harder when taking down stringer.
first up , think about your reason for using concave in relation to where you intend to start and finish the concave …nose ? mid ? tail ? …any concave will shorten the distance that water must travel from nose to tail, adding a touch of lift and speed . If it’s fared in from entry to exit , it will work …if it has flat spots particularly on entry and exit ,it can be a handbrake…keep it simple and with a predetermined purpose . Dead flat bottoms work great with plenty of rail edge , but very few shapers use them nowdays.